Optical fiber cable development, wherein the cable is capable of multi-channel transmission, has led to the use of bonded arrays of fibers which form a planar ribbon, and to stacks of ribbons within a core tube or sheath. In a typical ribbon array, a plurality of fibers, e.g., twelve or twenty-four, are held in spaced position parallel to each other by a suitable matrix, a configuration which simplifies construction, installation, and maintenance by eliminating the need for handling individual fibers. Thus, the splicing and connecting of the individual fibers can be accomplished by splicing and connecting the much larger ribbons provided that the fiber positions in the ribbon are precisely fixed and maintained.
In the prior art, numerous ribbon arrays have been proposed, all directed at achieving the aforementioned alignment as well as being directed to other aspects of ribbon construction and geometry. Among these is the adhesive sandwich ribbon (ASR) as disclosed in U.S. Pat. No. 4,147,407 of Eichenbaum, et al. and U.S. Pat. No. 3,920,432 of Smith. Such ribbon structures have proven to be easy to manufacture, rugged, and compact, and suitable for mass splicing. However, the use of adhesive tapes to form the sandwich necessitates a slow-down in the processing speed during manufacture and in an increased cost of the finished product, as well as the added loss to the fibers.
In U.S. Pat. No. 4,900,126 of Jackson, et al., the disclosure of which is incorporated herein by reference, there is shown a bonded optical fiber ribbon which comprises a coplanar array of longitudinally extending parallel optical fibers in contact with each other. Each fiber is enclosed in inner and outer layers of coating materials and has a color identifier coating for differentiating each fiber from the other fibers. The inner layer comprises an ultra-violet curable bonding material having a modulus of approximately 1 MPa and an outer layer of an ultra-violet curable bonding material having a modulus of approximately 1 GPa for mechanical protection. With the fibers disposed in a parallel array, interstices are created between the fibers themselves and between the fibers and the envelope of the ribbon, which is a matrix formed of an ultra-violet curable bonding material having a modulus that is less than the modulus of the outer coating layer on the fiber and which is greater than the modulus of the inner coating layer. The matrix material fills the interstices and bonds the fibers together and to the envelope to form a completed ribbon. The modulus and the surface characteristics of the matrix material and its bond to the color identifier on each fiber are such that interfiber and inter-ribbon movement can occur, thus the ribbons in a stack can slide relative to each other when the cable is bent and also that accessing of individual fibers is possible. The ribbons may be stacked such that eighteen ribbons, for example, having twelve fibers each, may be enclosed within a core tube to form the core of an optical fiber cable having two hundred and sixteen fibers, or, if preferred, channels. The core tube itself has an outside diameter (O.D.) of approximately 0.6 inches. Such an arrangement, which is in widespread use today, has proved adequate for most present day applications, but it imposes a definite upper limit on the numbers of fibers available and their individual identification and their accessibility.
There is, today, an ever-increasing demand for increased optical fiber cable capacity which is expected to continue into the foreseeable future. Higher fiber count cables and higher fiber packing densities are under constant and ongoing study and development. Extremely high fiber count cables have been proposed that use downsized fiber coating aimed at increasing packing density, however, the long term reliability, engineering, and operational characteristics are not, as yet, fully understood. Hence, an increase in the number of fibers, and, in turn, an increase in packing density, in a standard sized cable and with fibers having the standard thickness of fiber coating, is greatly to be desired. In an article entitled "A Modular Ribbon Design For Increased Packing Density of Fiber Optical Cables" by K. W. Jackson, et al., International Wire & Cable Symposium Proceedings 1993 at pages 20 through 27, the disclosure of which is incorporated herein by reference, there are given the results of a study of the feasibility of increasing the fiber packing density in a high fiber count cable. The cable design concept disclosed therein is based upon a modular structure of the ribbons used in the cable, and it is determined that the packing density for existing cable designs can be increased by as much as thirty to fifty percent. The ribbon structure proposed in that article comprises, for example, an array of sixteen fibers in side by side contacting relationship and divided, as by color coding of the fibers, into two eight fiber modules which, in turn, can be divided into four fiber modules. Each of the ribbons to be stacked within the cable bears, on its surface, identifying alphanumeric numbers. Thus, each fiber within each module in the stack is uniquely identified by two identifiers, i.e., color and ribbon number.
As pointed out in the aforementioned Jackson et al. patent, the color identifier material of each of the fibers should not be removed from the fiber when the bonding material is removed to access the fibers. Thus, the matrix material of the bonded ribbons is selected to have an interfacial bonding characteristic such that the bond interface of the matrix material to the coloring material is weaker than the bonding interface of the coloring material to the outermost coating on the optical fiber. In at least one embodiment of the invention of that patent, a release agent is applied over the coloring material prior to application of the matrix bonding material. There remains a problem, however, in breaking out separate modules from the ribbon, and individual fibers from the module. In high fiber count cables, where, for example, ribbons of twenty-four fibers are used, in most cases, it will be necessary or desirable to divide the ribbon into sub-units of twelve, six, four, or other numbers of fibers, for routing within the network or for achieving precise splicing. Inasmuch as it is the generally accepted practice, as pointed out, to have the bond between the matrix material and the coloring material, i.e. ink, weaker than the bond between the ink and the outermost coating of the fiber, dividing a ribbon into sub-units can result in sub-units of less than desired robustness. The edge fibers in a sub-unit thus produced, that is, those fibers adjacent the line of division, are exposed, and are not as firmly held by the matrix. Thus, they can separate from the ribbon sub-unit during handling. On the other hand, accessing individual fibers that are, for example, in the mid-span of the sub-unit, requires low adhesion between matrix and ink. In the past this dilemma has required a careful balancing of adhesions to achieve a measure of robustness while maintaining strippability for individual fibers.